1. Field of the Invention
The present invention relates to a steam moistening apparatus with a housing which includes a steam connection and in which a steam blow chamber is arranged, wherein the steam blow chamber has an external wall in common with the housing and the external wall is provided with steam outlet openings.
2. Description of the Related Art
Steam moistening apparatus of this type have the purpose of directing steam against material webs travelling past the apparatus in order to increase the moisture and temperature of the material webs. A widespread field of use is the manufacture or processing of paper webs in which such steam moistening apparatus are used in connection with calendars or other roll arrangements. In these arrangements, steam is directed against the paper web before the paper web travels through a roll gap, in order to improve the gloss or the smoothness, to change the bulk or the density or to increase the moisture.
A steam moistening apparatus known from DE 43 09 076 A1, constructed as a steam spray pipe, includes a steam blow chamber which is divided into several sections or zones in transverse direction, i.e, over the width extending in the direction of a material web travelling past the apparatus. Each zone has a valve which permits steam to flow from the interior of the housing into the steam blow chamber in each zone. An acceleration duct is arranged between the valve and the steam blow chamber, wherein a supply duct branches off from the steam blow chamber at a predetermined distance from the end of the acceleration duct.
Another steam moistening apparatus known from DE 41 25 062A1, constructed as a steam blow box, includes a steam blow chamber which is also divided into zones in transverse direction, wherein each zone has its own valve for admitting steam into the steam blow chamber. The steam entering the steam blow chamber had first been used for heating at least one of the walls of the steam blow chamber. Before the steam is used, it is dried in a steam drying section.
Steam moistening apparatus of this type have the advantage that the discharged steam quantity can be adjusted differently at least from zone to zone transversely of the machine direction. However, the apparatus have the disadvantage that they are complicated and, thus, expensive because of the large number of valves of the moistening apparatus. If a single moistening apparatus is not sufficient for directing steam against a side of the material web, for example, when operated at the limit of its capacity, frequently two or more moistening apparatus are used. However, in that case, it is no longer necessary to equip all moistening apparatus with separate controllable zones. Rather, it is sufficient when it is possible to change the total quantity of steam discharged by the steam moistening apparatus. It is then possible to achieve a uniform discharge of steam in transverse direction of the machine, i.e., transversely of the travelling material web, by means of a single steam moistening apparatus which can be controlled zone by zone.